This invention relates to improvements in electrochemical sensors of the general type disclosed in Clark, Jr. U.S. Pat. No. 2,913,386. It is useful for measuring the dissolved concentration of any of a wide range of electro-reducible or electro-oxidizable gasses in a rapid and accurate manner and finds particular utility in applications such as environmental monitoring wherein large geographical areas must be observed for identification of specific sources of pollution. For example, a sensor constructed in accordance with the present invention may generate data for a map of dissolved oxygen concentrations, while being towed around a harbor or up a moving stream.
Sensing of a dissolved gas in accordance with the teachings of Clark proceeds by positioning an anode and a cathode inside a suitable probe and placing them in contact with an electrolyte solution. The electrolyte solution is sealed against the electrodes by means of a membrane which is permeable to the gas being monitored. When the probe is immersed in a sample of the source liquid, the gas permeates the membrane and undergoes an electro-reaction at one of the electrodes while a balancing electro-reaction takes place at the other electrode. This causes an electrical current to flow from one electrode through the electrolyte and into the other electrode. Clark found that a voltage of about 0.6 volts should be maintained across the cell in order to provide the required electrode potential for measurement of oxygen.
When used in connection with the detection of oxygen, the following overall reaction takes place within Clark's cell: EQU 4H.sup.+ +O.sub.2 +4e.sup.- .fwdarw.2H.sub.2 O
In general, the substance being measured tends to depolarize the cell. When the needed ions are available and a minimum polarizing potential is maintained across the cell, then the current flow through the electrodes is proportional to the rate at which dissolved gas is passing through the protective membrane.
Clark's teachings have provided a basis for numerous sensor probes developed over the years. Many of the developments have concentrated on improving the accuracy and the response time of the basic Clark probe and have provided a variety of techniques for measuring the current flow through the electrodes. In a paper entitled "Determination Of Oxygen Tension By Measurement Of Net Charge Transport", IEEE Transactions On Biomedical Engineering, November 1976, pages 472-477, Gregory L. Zick taught that the polarizing potential may be pulsed and that the current flow through the electrodes may be integrated during portions of the charging and discharging cycles to determine net charge. His circuit amplified the current flow, applied it to an A/D converter and then applied the digitized current sample to a mini-processor for waveform integration.
Zick applied the above teaching to a transcutaneous electrochemical cell which was patented under U.S. Pat. No. 4,269,684. Zick's patent discloses a computer which stores samples of the polarizing and depolarizing waveforms resulting from a polarizing pulse. The areas under the two waves are determined by digital integration, and an uncorrected value of the oxygen partial pressure is established by calculating the difference between the two waveform areas. The uncorrected partial pressure measurement is corrected for electrode drift by comparing discharge waveforms of successive pulses. FIG. 5 of the patent illustrates an analog circuit for integrating his polarizing and depolarizing waveforms.
Langdon U.S. Pat. No. 4,556,472 teaches a microcomputer system using a Clark-type sensor for sensing oxygen concentration in a pulsed fashion. Langdon also measures current flow. His system employs real time signal averaging to reduce the noise level and improve precision. He teaches the use of his device for environmental sampling and states that for oxygen, a pulse duration of from 0.5 to 3 seconds and a pulse interval of from 3 to 5 minutes or more are suitable.
Mund et al U.S. Pat. No. 4,853,091 discloses a miniaturized oxygen sensor for use with an implantable pacemaker. No diffusion membrane is employed. The disclosed device includes an arrangement of three electrodes and means for generating a pulsed polarizing voltage. One of the three electrodes functions as a reference electrode for making measurements of current flowing between the other two electrodes. The cycle time is between 0.5 and 10 seconds, and the current is integrated following a delay of 2 to 40 msec. The patent teaches that the measuring signal is not influenced by the capacitive current required for charging the double layer. A teaching of double layer physics is set out in Pons et al U.S. Pat. No. 4,783,250.
In spite of Clark's teachings and the various above-discussed extensions thereof, no fully satisfactory electrochemical sensor has been developed for long term environmental monitoring and similar applications. Currently available sensors are unable to meet harsh environmental conditions, or are deficient in speed or accuracy, or are excessively expensive. A further common problem is a high rate of gas consumption, which has necessitated either a long wait between measurements for electrode recovery or recourse to a stirring device with its attendant mechanical problems and consumption of power.